Spontaneous preterm birth: a case-control study.

Information on demographic characteristics, life-style, and reproductive, prenatal, and medical history was obtained from the prenatal records of 140 women with a preterm delivery (cases) and 280 women with a term delivery (controls) matched by age. Compared with controls, a greater proportion of cases were nonwhite, never married, and educated less than 12 years. When logistic regression was used to control for confounding variables, a number of risk factors were found to be associated with an increased risk for preterm delivery: a history of a prior preterm delivery (relative risk, 3.5; confidence interval, 1.6 to 7.8), smoking during the pregnancy (p value test of trends, 0.005), and a prepregnancy weight less than 61.5 kg (relative risk, 2.0; 95% confidence interval, 1.2 to 3.2). Seven (5%) cases and none of the controls indicated a history of maternal diethylstilbestrol exposure (p less than 0.001). A history of prior induced abortion was associated with a modest increase in risk for a spontaneous preterm delivery (relative risk, 1.6; confidence interval, 0.9 to 2.7), and this risk increased with increasing numbers of induced abortions (p value for chi 2 test of trend, 0.02).